260 GENERAL ZOOLOGY 



tomical and not complete at that, because no gill capillaries are 

 present. Since the gill clefts of the chick later on disappear, 

 accompanied by changes in the branchial circulation, it would 

 seem that the only reason for their development at all in the chick 

 is that the chick in its development is merely following the path 

 along which its evolution from lower forms took place. The 

 fact that the chick develops gill clefts and aortic arches seems to 

 mean that the chick is descended from ancestors in which these 

 structures had a functional significance. Hence the appearance 

 in an embryo of a higher form of morphological relationships 

 characteristic of lower forms is taken as an indication of rela- 

 tionship between the higher and lower forms. A more accurate 

 picture of this relationship is obtained if corresponding embryonic 

 stages are compared. Thus the resemblance in the branchial 

 region is much closer between the chick embryo and the fish 

 embryo than between the chick embryo and the adult fish. 



No animal in its development repeats every step in its racial 

 history or phylogeny — many are slurred over and omitted, and 

 new ones interspersed among the old — so that a general conclu- 

 sion must be limited to the statement that ontogeny is, to a 

 certain extent only, a repetition of phylogeny. This generaliza- 

 tion, known as the law of bioge?iesis, seems to hold throughout 

 the animal kingdom and has been useful in many cases in throw- 

 ing light upon phylogenic relationships. 



Homology. — A practical difficulty in applying the biogenetic 

 law as a criterion for determining the origin of embryonic con- 

 ditions is that in the embryo old (paling enetic) characteristics 

 are intermingled with new (cenogenetic) ones, so that an embry- 

 onic structure is never a perfect recapitulation of an ancestral 

 structure. Thus the circulation of the vertebrate embryo only 

 approximates the fish type of circulation; but the inference is 

 implied that the fish type of circulatory system is near to the 

 primitive type from which the circulatory systems of the higher 

 vertebrates have evolved. The wings and legs of birds and the 

 limbs of quadrupeds and of man all develop from the same sort 

 of embryonic limb rudiment, which in its inception is an out- 

 growth of the body wall. The development of vertebrate limb 

 buds may be said to be practically the same for all vertebrates 

 up to a certain point, beyond which the path of development 

 diverges in various directions. Vertebrate limbs are said to be 



